the.com/open letter

means A letter addressed to one person or group but deliberately published for everyone to read.

from The format is old — writers and officials have published persuasive letters-to-one-but-meant-for-all since at least the 17th century, with the Latin tradition of 'epistola' written for circulation standing behind it. The phrase 'open letter' itself emerged in English print culture (newspapers and pamphlets) as a simple, literal label: a letter left 'open,' unsealed, for public eyes rather than sealed for one recipient alone — the opposite of a private, folded, wax-stamped letter meant for no one else's view.